Automotive air conditioning systems, like all air conditioning systems, are faced with a number of operating contradictions. These contradictions include a requirement to provide cooling, but not too much cooling, in the passenger compartment. There is a technical requirement to lubricate the refrigerant compressor, but not to foul downstream heat exchangers with the lubricant. In automotive systems, additionally, consumers expect instantaneous response in the passenger compartment to what may be a very large and very rapidly changing heat load. Of course, while the only power available is that supplied by the engine and the automotive battery, automotive consumers also expect that operation of the air conditioning system will not load the engine or cause any operating difficulty. Consumers also expect that the automotive air conditioning system will have low power consumption.
Traditional automotive air conditioning systems used a clutch, in which the air conditioning compressor was engaged or disengaged to provide power to the compressor and thus supply cooling to the passenger compartment. Of course, the on/off nature of this control provided slow response. The prior art tried to meet the needs described above in a variety of ways, principally by using a variable displacement compressor. In clutchless variable displacement compressors, the compressor is always on, i.e. always rotating, while the displacement of the compressor is determined by the angle at which a central swashplate is oriented to a number of pistons and cylinders in which refrigerant compression takes place. A narrow angle (perpendicular to a drive shaft) provides little compression, while steep angles (at some angle to the drive shaft) provide greater compression, depending on the angle selected. However, some present variable displacement compressors allow too much oil into the downstream air conditioning components, such as the gas cooler or condenser, and the evaporator, fouling their internal surfaces and reducing heat transfer to the passenger compartment. In addition, high loads on the compressors can load down engines, in extreme cases causing stalling in awkward situations. Finally, the response time for systems using variable displacement compressors can be long, resulting in longer cooling cycles and higher power consumption than necessary. What is needed is a control system that responds rapidly to air conditioning loads and minimizes oil contamination and energy consumption, without loading the engine or causing stalling.